Viscoelastic icephobic surfaces

ABSTRACT

Viscoelastic icephobic surfaces of the present disclosure include organogel particle beads dispersed in an elastomer matrix. The surfaces are highly repellant to ice formation, easy and cost efficient to apply, and have long term durability for harsh outdoor applications.

BACKGROUND OF THE INVENTION

This application is a continuation of and claims priority to U.S. Pat.Application No. 16/754,505, filed Apr. 8, 2020, entitled “ViscoelasticIcephobic Surfaces,” which is a 371 application of PCT/US18/55793, filedOct. 15, 2018, which claims priority to U.S. Provisional Pat.Application No. 62/572,708, filed Oct. 16, 2017, entitled“Nano-Viscoelastic Anti-Icing Surfaces,” the entire contents of whichare hereby incorporated by reference.

This disclosure pertains to anti-icing surfaces that are ice phobic aswell as flexible, durable, and useful for a variety of applications.

Anti-icing surfaces play a critical role in a broad range of systemsincluding infrastructure and energy systems. In cold climates and duringwinter storm events, the absence of these surfaces can lead tocatastrophic failures in power systems (e.g. power towers, powerstations, and power lines), transportation systems (e.g. aviationindustry and ocean-going vessels) and energy systems (e.g. domestic andlarge power plants). According to Lawrence Berkeley Laboratory, icestorms account for 10% of power transmission outages in the U.S. Thefinancial loss is approximated as $3-5 billion annually. In addition tofinancial losses, around 3 million people in the U.S. every wintersuffer from power losses caused by ice storms. Icing may lead tocollapse of poles and towers and rupture of conductors. In the aviationindustry, icing on aircrafts results in increased drag and loss of liftforce, potentially leading to catastrophic events. Icing in coolingsystems significantly drops the heat transfer rate and leads toinefficient operation of these systems.

The main figures of merit for ice phobic surfaces are low freezingtemperature, low ice accretion rate, and low ice adhesion. Furthermore,long-term durability of these surfaces is another critical factor.Multiple products (e.g. super hydrophobic, non-wetting, liquid-infused,and hydrated surfaces) have been developed to reduce or prevent iceaccumulation. However, high ice adhesion strength (~20-100 kPa) andsubsequent ice accretion, low long-term mechanical and environmentaldurability, and high production cost have restricted their applications.

SUMMARY OF THE INVENTION

The present disclosure relates generally to surfaces that are icephobic. In particular, the ice phobic surfaces are nano-viscoelasticsurfaces that are spray-able, flexible, durable, and universal in theirapplications. The nano-viscoelastic surfaces have unprecedentedanti-icing characteristics, are low in cost, and are resistant to highshear flows.

Generally, the present nano-viscoelastic surfaces can be prepared by (1)developing silicon-based organogel particle beads, (2) mixing theparticle beads with a surfactant and crushing the mixture to reduceaggregation, (3) separately preparing an elastomer matrix having a highshear modulus from suitable types of elastomers, such as siliconeelastomers, to serve as a host for the developed gel beads, (4)incorporating the developed organogel particle beads prepared in step(2) within the prepared elastomer matrix, and (5) applying the finalmixture to any surface and letting the mixture cure at room temperatureto obtain the final nano-viscoelastic icephobic surface.

The nano-viscoelastic anti-icing surfaces are stable under high shearflows and high and low temperatures. The surfaces demonstrate enhancedanti-icing characteristics with long term durability for harsh outdoorapplications. A new physical concept called stress-localizationcontributes to the effectiveness of the present icephobic surfaces,which have exceptional mechanical, chemical and environmentaldurability. The concept of stress localization reduces ice adhesion onthese materials by an order of magnitude and is far more effective thanpreviously studied surface-modified methods. Stress-localization is avolumetric phenomenon and remains effective even after long duration ofoperation of these materials. Furthermore, the icephobic material doesnot affect aerodynamic characteristics of airfoils offering a promisingsolution for aerospace applications.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A shows a schematic of ice detachment from a material with anicephobic coating.

FIG. 1B shows a schematic diagram of stress-localized icephobic coatingsin accordance with preferred embodiments described herein.

FIG. 1C shows a surface map of elastic modulus of stress-localizedicephobic coatings in accordance with preferred embodiments describedherein.

FIG. 1D shows a schematic of a formation of a crack and detachment ofice at a Phase II coordinate in stress-localized icephobic coatings inaccordance with preferred embodiments described herein.

FIG. 2 shows a schematic of a nano-viscoelastic and anti-icing surfaceprepared in accordance with preferred embodiments described herein.

FIG. 3 shows images of sample nano-viscoelastic surfaces before andafter submersion in solvents, with a scale bar of 10 mm.

FIG. 4 shows images of a sample nano-viscoelastic surface before andafter 1000 cycles of an abrasion test, with a scale bar of 10 mm.

FIG. 5 shows images of a sample nano-viscoelastic surface before andafter UV radiation exposure for 500 hours, with a scale bar of 10 mm.

FIG. 6 shows results of ice adhesion tests for various samplenano-viscoelastic surfaces measured at - 15° C.

FIG. 7 shows results of ice adhesion tests for a samplenano-viscoelastic surface at various surface temperatures.

FIG. 8 shows a schematic of an anti-icing process on a samplenano-viscoelastic surface before and after applying a 17 m/s wind.

FIG. 9 shows a schematic of a setup for ice adhesion measurements.

FIG. 10A shows ice adhesion values for state of the art technologycompared to stress-localized icephobic coatings in accordance withpreferred embodiments described herein.

FIG. 10B shows ice adhesion values for various concentrations of PhaseII gel particles and various icing/deicing cycles for stress-localizedicephobic coatings in accordance with preferred embodiments describedherein.

FIG. 10C shows ice adhesion values for stress-localized icephobiccoatings in accordance with preferred embodiments described herein asprepared and following exposure to water and air shear.

FIG. 10D shows ice adhesion values for stress-localized icephobiccoatings in accordance with preferred embodiments described herein asprepared and following exposure to chemical environments and UV.

FIG. 11A shows thickness of state of the art technologies compared tostress-localized icephobic coatings in accordance with preferredembodiments described herein following abrasion.

FIG. 11B shows ice adhesion of state of the art technologies compared tostress-localized icephobic coatings in accordance with preferredembodiments described herein following abrasion.

FIG. 12A shows a schematic of an experimental setup to evaluateaerodynamic properties of icephobic coatings in accordance withpreferred embodiments described herein.

FIG. 12B shows drag coefficients for wings coated with exemplaryicephobic coatings and uncoated wings as a function of angle attack.

FIG. 12C shows lift coefficients for wings coated with exemplaryicephobic coatings and uncoated wings as a function of angle attack.

FIG. 12D shows ratio of lift/draft of airfoils coated with exemplaryicephobic coatings and uncoated airfoils as a function of angle attack.

FIG. 13A shows a schematic of an experimental setup to evaluate crackinitiation in ice found on Phase II coordinates in exemplary icephobiccoatings.

FIG. 13B shows images of interfacial cavities in ice formed at Phase IIcoordinates in exemplary icephobic coatings obtained through opticalmicroscope and high-speed imaging.

FIG. 13C shows stress localization as a function of concentration ofPhase II particles in exemplary icephobic coatings.

DETAILED DESCRIPTION OF THE INVENTION

The present disclosure relates to anti-icing surfaces, and particularlyto anti-icing surfaces that have nano-viscoelastic characteristics andare highly flexible, durable, and universal in application.

The present icephobic material shows extremely low ice adhesion whilehaving long-term mechanical, chemical and environmental durability. Theicephobic material, stress-localized viscoelastic material, utilizeselastic energy localization at the ice-material interface to shear theinterface. With minimal applied force, cracks are formed at theinterface generating local stress fields. This shear stress advancescracks at the interface to detach ice form the material. This icephobicmaterial is a smooth coating and would not affect the aerodynamicproperties of a surface such as airfoil.

Once ice forms on a surface, the interaction between ice and thesubstrate is governed by van der Waal’s force, electrostatic forces orhydrogen-bonding forces. A wide range of surfaces has been studied toreduce ice adhesion strength. Among those, elastomers have shown minimumice adhesion and have the potential to achieve exceptional icephobicproperties. Consider a rigid ice phase attached to an elastomer as shownin FIG. 1A. If a shear force is applied in the ice-elastomer plane, theice would only slide with no detachment from the surface. However, ifthe force is applied at a plane higher than the interface, the ice woulddetach at a critical stress. It has been shown that the elasticinstability at the interface of a rigid body and an elastomer isresponsible for fracture. The fingers developed at the contact line byelastic instability elongate and break down in the form of bubbles thathelp in propagation of crack at the interface. The threshold for bubbleformation depends on the shear modulus of the elastomer. For a uniformelastomer with isotropic properties, one finds that the adhesion stressat the interface (σs) is written as:

$\sigma_{s} \cong \left( \frac{a}{l} \right)\sqrt{\frac{W_{a}G}{h}}$

where a and l are the geometrical parameters as shown in FIG. 1A, W_(a)is the work of adhesion, G is the shear modulus, and h is the thicknessof the elastomer. This formulation suggests that low ice adhesion can beachieved through low values of G and W_(a). Note that the value of G canbe tuned by several orders of magnitude, but the value of W_(a) in thebest case can be tuned by an order of magnitude (e.g. introduction ofperfluorinated groups on a surface). By tuning the substrate from hardelastomers (G~ 1 GPa) to gel (G~ 1 Pa), low values of ice adhesion hasbeen achieved. However, low values of G lead to low mechanicaldurability of the icephobic coatings, which results in poor long-termperformance. The values of a, l and h are determined by dimensions ofexperimental instrument and icephobic material. Inconsistency in thesedimensions in measuring of ice adhesion has resulted in scattered dataof ice adhesion for the same substrate. For example, the reported valuesof ice adhesion for PDMS varies in the range of 100-800 kPa.

A standard method to measure ice adhesion is described in Example 2below. In the above formulation, an isotropic elastomer was considered,which resulted in a direct dependence of σS on G. However, once localphases with low shear modulus, such as those in the present icephobicsurfaces, are introduced at the ice-material interface, as shown in FIG.1B, with minimal force, ice is detached from local phases and forms alocal crack. This local crack induces an elastic stress field around thecrack. This induced shear stress field opens the crack front and leadsto propagation of crack at the interface. That is, the induced stressfield by local phases leads to crack growth and failure. Throughmathematical formulation of the discussed physics, the ice adhesionstrength on these surfaces is written as

$\left. \sigma_{s} \right.\sim g\left( \varphi_{\Pi} \right)\left( \frac{a}{l} \right)\sqrt{\frac{\overline{W_{a}}G_{m}}{h}}$

where g(φ_(II)) denotes the stress-localization function, φ_(II) is thevolumetric fraction of phase II, W_(a) is the work of adhesion of thematerial, and G_(m) is the shear modulus of the material. The values ofW _(a) and G_(m) depend on properties of individual phase I and II,their volumetric fraction and their geometry. The salient feature ofthis formulation is the stress-localization function, which plays acritical role in the adhesion of ice to the material and its impact isfar more effective than other parameters studied before (i.e. work ofadhesion and shear modulus). This localization function reduces theadhesion of a solid on an elastomer by an order of magnitude asdemonstrated and discussed below.

Based on the developed stress-localization concept, a new form oficephobic surface, stress-localized viscoelastic material was developed.The material includes a matrix as Phase I with high shear modulus andhighly dispersed phase II with low shear modulus. An exemplary procedurefor development of one embodiment of these materials is given in Example2 below. Phase I is a silicon elastomer and Phase II is a silicon-basedorganogel. As the matrix of this material plays a major role inlong-term mechanical durability, it is crucial to choose an elastomerwith high shear modulus. The preferred silicone elastomers are roomtemperature vulcanizing (RTV) with certain mechanical properties. Toform a homogenous material, compatibility of the matrix and thedispersed phase is critical. Thus, silicon-based organogel particleswith dimension of 2-20 µm are preferred. Other combinations ofelastomers and the dispersed phases may be used as long as they providea homogenous material. Once the material is developed, its viscosity canbe adjusted through a solvent. Here, hexamethyldisiloxane is used inpreferred embodiments to reduce the viscosity of the material. In thedilute form, the material can be brushed or sprayed to form a uniformcoating. Once applied, the material is completely cured after 24 hrs.The surface of these materials was examined through Scanning ProbeMicroscopy (SPM) (Bruker Multimedia 8 SPM) to determine distribution ofPhase II on the surface. FIG. 1C shows modulus of elasticity of bothphases. As shown Phase II has much smaller modulus than that of thematrix. FIG. 1D shows a representation of a formation of a crack at acoordinate of Phase II with minimal forces.

FIG. 2 shows a schematic of a viscoelastic anti-icing surface preparedin accordance with preferred embodiments of this disclosure. Theicephobic surface includes a phase of organogel particles (also referredto as Phase II) dispersed throughout an elastomer matrix (also referredto as Phase I). Preferred concentrations of organogel particles in theelastomer matrix are about 1% to about 99% based on volumetric ratio,and more preferred concentrations are about 5% to about 85%. The PhaseII particles should be generally dispersed throughout the elastomermatrix to avoid accumulation of particles in isolated regions.

The viscoelastic icephobic surfaces can utilize a variety of differentelastomers that serve as a host or matrix. In certain embodiments theelastomer can be a room-temperature-vulcanizing (RTV) silicone rubberprepared using a suitable base and a curing agent. Additional preferredelastomers may include polyurethane, poly isoprene, fluoroelastomers,and the like. The selected elastomer should have a high shear modulus.

Different types of gels can also be used as Phase II particle beads tobe integrated within the elastomer matrix. The gel beads may be made oforganogels (gels made of hydrocarbons), polyacrylamide,polydimethylsiloxane (PDMS), or other suitable materials. The gel beadsmay be mixed with a variety of different surfactants, including butylbutyrate, propylene glycol, and silicone (Si) oil, and crushed prior toincorporation into the rubber or polymer matrix. In preferredembodiments, the organogel particles include tuned liquid organic phases(non-crosslinked components in the gel matrices) entrapped within asolid phase (three-dimensionally crosslinked gel network). In certainpreferred embodiments, the organogel particles are made up ofcombinations of siloxanes, silicas, and ethyl benzene. In additionalpreferred embodiments, the organogel particles are made up of acombination of dimethyl siloxane, dimethylivinyl terminated silica,dimethylvinylated silica, trimethylated silica, tetra (trimethoxysiloxy)silane, ethyl benzene), dimethyl, methylhydrogen siloxane, andtetramethyl tetravinyl cyclotetra siloxane. In additional preferredembodiments, the organogel particle beads are polydimethylsiloxanebased. The gel beads incorporated into the elastomer matrix arepreferably about 10 nm to about 100 microns in diameter, and morepreferably about 2 to about 20 microns.

Generating the nano-viscoelastic surfaces from different materialsallows for alteration of the properties of the product, which alsoallows for adjusting the desired durability and ice adhesion propertiesbased on the desired function for the surface. Table 1 below illustratessome types of materials that may be used to develop preferredembodiments of the nano-viscoelastic surfaces.

Table 1 Matrix Gel beads Polyurethane PU1 PU2 PU3 Poly isoprene PO1 PO2PO3 Silicone rubber SI1 SI2 SI3

The nano-viscoelastic anti-icing surfaces are physically and chemicallystable while maintaining extremely low ice adhesion properties. Inpreferred embodiments, the nano-viscoelastic surfaces are applied to asurface in need of protection from icing by spraying the uncuredmaterial to the base surface and allowing the material to cure to formthe anti-icing surface. An important factor for the long term durabilityof anti-icing surfaces is their ability to adhere to the surface andalso their ability to withstand severe abrasion. These factors becomeincreasingly relevant for outdoor operation. Most current anti-icingtechnologies cannot demonstrate this physical stability for prolongeddurations. The current nano-viscoelastic surfaces have been tested andverified to have physical stability in these conditions.

The present anti-icing surfaces are highly durable icephobic materials.These materials utilize stress-localization to initiate cracks at theice-material interface and consequently minimize ice adhesion on thesurface. Stress-localization leads to a shear force at the interface fordetachment of ice from the material. The developed concept isimplemented in elastomers and the superior icephobicity of thesematerials compared to state-of-the-art materials is demonstrated. Theseforms of icephobic materials demonstrate excellent mechanical, chemicaland environmental durability with no change of characteristics underextreme air and water shear flows. Furthermore, these icephobicmaterials do not change the aerodynamic characteristics of airfoilsthereby providing a promising solution for aerospace application. Incontrast to surface modified coatings, the icephobicity of thesematerials is a volumetric property and no degradation in the performanceoccurs in long-term operation under mechanical loadings. The developedconcept of stress-localization reduces adhesion of solids on a materialby an order of magnitude with no compromise in mechanical properties.The developed icephobic materials could be used to minimize adhesion ofany solid species (i.e. ice, gas hydrate, dust, and even bio-species) ona surface with omnipresent application in transportation systems(aviation, cars and vessels), energy systems, and bio-sciences.

Example 1

To verify the properties of the nano-viscoelastic surfaces, testing wascarried out on a preferred embodiment identified in Table 1 above asSI3. Sample SI3 was created by preparing polydimethylsiloxane (PDMS)beads (SYLGARD® 184, The Dow Chemical Company), then mixing and crushingthe PDMS beads in a silicone (Si) oil surfactant until the beads arenano-micro sized, or about 10 nm to about 200 microns. A polymer base ofsilicone rubber was separately prepared and the crushed beads were addedto the silicone rubber base. Prior to curing, a portion of the polymerbead mixture was applied to a surface made of glass at a thickness ofabout 400 microns and a width of about 25 mm and a length of about 70mm, then allowed to cure for 30 minutes to prepare a SI3 sample surface.

To test chemical stability, the SI3 sample surfaces were submerged inseparate containers containing the solvents alcohol, acetone, or tolueneat room temperature overnight. FIG. 3 shows the sample products beforeand after submersion overnight. No changes were observed on the surfacesafter complete submersion in the chemicals overnight. Thus, sample SI3was chemically inert to these materials, demonstrating long termdurability.

An abrasion test was carried out with 2 newton force directly appliedonto the surface of the SI3 sample using linear TABER® Abraser equipment(Taber Industries, New York, USA) with CS-10 as the fine abrader andH-18 as the medium abrader. The as-prepared sample was clamped down andtested for 10,000 abrasion cycles. FIG. 4 shows the SI3 sample surfacebefore and after running the abrasion test. Only 97 microns of thesurface was removed, which is considerably less material loss comparedto current state-of the art technologies, thereby proving physicalstability and durability.

To further evaluate the physical durability of these anti-icingsurfaces, sample SI3 was tested for UV radiation effects. The sample wasplaced in a fluorescent chamber for 500 hours to be fully exposed to UVradiation. FIG. 5 shows the SI3 sample before and after spending 500hours in a UV radiation chamber with a wavelength of 250-400 nm and alamp power of 40 W. After removing the sample from the UV chamber, nocracks or material degradation were spotted. The sample was thenre-examined using the abrasion test under 2 N force after UV radiationexposure. The amount of material removed from the product was 101microns, again a very small amount of material.

The anti-icing characteristics of the sample surfaces identified inTable 1 were also studied. Sample surfaces were generally preparedaccording to the details provided above for the preparation of the SI3sample surface, using different gel beads, surfactants, and polymer, toprovide similarly sized sample surfaces. The ice adhesion strength onthe sample surfaces was measured through direct applied shear stress. Inthis approach, a rectangular cuvette was placed on the cold sample. Thecuvette was filled with water for ice formation on the sample. Theformed ice was left for 1 hour on the surface before the measurement. Ashear force was applied tangentially to the ice cube and measured usinga digital force gauge such as the IMADA DS2-110 (Imada, Inc.,Northbrook, Illinois) to determine the detachment force required toremove the ice from the surface. The detachment force divided by theice-sample surface area gave the ice adhesion strength. FIG. 6 shows theresults of ice adhesion measurements taken at - 15° C. for varioussample surfaces shown in Table 1. Sample surface SI3 provided aconsistent average ice adhesion of 4.5 kPa ± 2 at -15° C. which wasindependent of the number of icing/de-icing cycles. This alsodemonstrates the durability of these surfaces for anti-icingapplications. The anti-icing properties of sample SI3 were alsore-examined after the abrasion test and the average ice adhesion wasstill found to be 4.5 kPa ± 2 at -15° C.

Multiple ice adhesion measurements were taken for sample SI3 to evaluatethe performance of the sample at different temperatures using theprocess described above. FIG. 7 shows the average ice adhesionmeasurements for sample SI3 at different surface temperatures. Even atvery low temperatures of - 30° C., the ice adhesion strength on thesample surface was relatively low.

Additional tests were also carried out on sample surfaces identified inTable 1. The mass change of the surfaces was studied several monthsafter the surfaces were prepared, and no sign of any mass change wasobserved. The sample surfaces were also stored at 100° C. for more than24 hours, which no change in mass or other characteristics observedfollowing this heat treatment. The sample surfaces were also stored at -30° C. for more than 5 hours to measure shrinkage effects, but theresults were found to be negligible. Low ice adhesion properties weredemonstrated in sample SI3 by inducing a 17 m/s average air velocityacross the surface having ice droplets. FIG. 8 shows a schematic of thesample surface having an ice droplet before applying the wind, thenafter applying the wind at 17 m/s.

Example 2

Exemplary stress-localized icephobic materials were developed. Phase I,the elastomer, was a RTV-1 silicone rubber. The RTV-1 silicone rubberhad the material properties of: Elongation at break - 500%, HardnessShore A - 30, Tensile strength - 8 N/mm², Viscosity, dynamic at 20° C. -300000 mPa.s, Density at 23° C. in water - 1.1 g/cm³, andtear-strength - 13.5 N/mm.

Phase II, organogel particles, consisted of tuned liquid organic phases(non-crosslinked components in the gel matrices) entrapped within asolid phase (three-dimensionally crosslinked gel network). The procedurefor development of these organogels was: 10 mL of base (SYLGARD 184, DowCorning – Dimethyl siloxane, dimethylivinyl terminated,Dimethylvinylated and trimethylated silica, Tetra (trimethoxysiloxy)silane, and Ethyl benzene) was mixed with 1 mL of curing agent (SYLGARD184, Dow Corning - Dimethyl, methylhydrogen siloxane, Dimethyl siloxane,dimethylvinyl terminated, Dimethylvinylated and trimethylated silica,Tetramethyl tetravinyl cyclotetra siloxane, and Ethyl benzene). 100 mLof an organic liquid (i.e. Polydimethylsiloxane (PDMS), or silicone oil)was added to this mixture. The solution was then vigorously mixed toobtain a homogeneous solution. The precursor sample was heated at 100°C. for 4 hrs in a petri dish. The final product was a non-syneresisorganogel. Non-syneresis property of organogel comes from miscibility ofthe components and silicone oil with PDMS before and after gelation.Generally, the organogel particles are made up of a cross-linkedpolydimethylsiloxane network with entrapped silicone oil.

Once phase II was developed, it was crushed in the presence of siliconeoil for ten minutes to avoid aggregation of gel particles. The solutionwas filtered to remove excess oil. The final product was a batch of gelparticles with dimension in the range of 2-20 µm. The particles weremixed with the elastomer in a pre-defined concentration, preferablyabout 1 to 99% based on volumetric ratio. The solution was diluted witha solvent, hexamethyldisilaxane, to reduce viscosity for spraying on asurface.

A standard procedure to examine ice adhesion on various materials wasdeveloped and utilized. Standard protocol was followed for all themeasurements. The schematic of experiments is shown in FIG. 9 . The testchamber was cooled at a rate of ~2° C./min to the target temperature.Temperature of the cooling plate was monitored using a thermocouple ontop of the plate. Four exemplary types of icephobic materials werecreated through tuning the volumetric ratio of phase II in the material.AI-10, AI-11, AI-12, and AI-13 stand for 67%, 50%, 33%, 25% of phase II,respectively.

The icephobic sample was placed on the cooling plate. A square acryliccuvette with dimension of 15 mm by 15 mm was fabricated with lasercutter with an accuracy of 100 µm. The edges of cuvette were coated withSilane in order to achieve low surface energy and minimize adhesion ofcuvette to the icephobic surface. This step minimizes the errors in iceadhesion measurements. The cuvette was filled with deionized water andwas allowed to freeze for 1 hr. Ice column encased in acrylic columnswas adhered to the test samples. The force required to detach each icecolumn was measured by propelling the 0.8 cm diameter probe of a forcetransducer (Imada, model DS2-110) to the side of the ice columns at aconstant velocity of 0.1 mm/s. The probe velocity was controlled using asyringe pump. The center of probe was located at 1 mm above the materialsurface. The measured maximum force at break was converted into iceadhesion strength by dividing by the known cross-sectional area (2.25cm²) of the ice-substrate interface. The entire experiment was conductedin a low-humidity nitrogen atmosphere to minimize frost formation on thesamples and the test apparatus.

The measured values of ice adhesion at temperature of -25° C. on allthese samples are shown in FIG. 10A. With the same experimentalprotocol, ice adhesion was measured on other state-of-the-art icephobiccoatings and included in FIG. 10A. The reported value of ice adhesion(σ_(S)) was the average of ten measurements. In the protocol of iceadhesion. All the samples had the same thickness of 300 + 20 CD. Asshown, ice adhesion on AI-10 is an order of magnitude lower than otherstate-of-the-art surfaces. This low ice adhesion is believed to beachieved through stress-localization. Another important metric forassessment of ice adhesion on coatings of uniform thickness is iceadhesion reduction factor (ARF) which is defined as ARF = σ_(S) (Al)/σ_(S) (icephobic material). This criterion is a non-dimensional figureto determine ice adhesion, independent of geometry of measurement setup.The ARF values for various samples are included in FIG. 10A, showingthat AI-10 reduces ice adhesion by 800 times compared to Aluminumsubstrate.

For some of the state-of-the-art materials, ice adhesion depends on thenumber of icing/deicing cycles as the properties of these materials(i.e. surface characteristics) changes. For example, for liquid-infusedsurfaces, the depletion of liquid on the surface adversely affectscyclic ice adhesion. For the developed stress-localized icephobicsurfaces, ice adhesion up to 100 icing/deicing cycles was determined.For these experiments, once the ice column was detached from thesubstrate, a new cuvette was placed on the sample and the procedure forice formation was repeated. After complete formation of ice column,standard procedure was followed to measure ice adhesion. For the samesample, these experiments were conducted up to 100 times during a weekto demonstrate consistency of ice adhesion on these icephobic surfacesand no change was observed. These experiments were conducted for variousgrades of these materials. FIG. 10B shows results.

To assess ice adhesion of these materials in harsh environments, theicephobic coating was exposed to high shear flow of water and air up toReynolds number of 2 × 10⁴ and 3 × 10⁴ respectively for one month. Forthese experiments, icephobic material was coated on a glass substratethrough spraying. The sample was left to cure for 24 hr. The iceadhesion on the sample was measured through the protocol describedabove. Next, the coated glass substrate was placed in a tube andinitially was exposed to shear flow of water with Reynolds number of20000. The sample was left under high shear flow for one month. Afterthis time period, the ice adhesion on the sample was re-measured. Thesame sample was moved to another setup and was exposed to shear flow ofair with Reynolds number of 30000 for one month. The ice adhesion on thesamples was remeasured after this experiment. No change in the iceadhesion was observed. FIG. 10C shows results.

To resemble samples exposed to various chemical environment, theicephobic samples were exposed to solutions with pH ranging 1-13 andre-examined using the standard ice adhesion protocol. FIG. 10D showsresults. Furthermore, to demonstrate long-term ice adhesion of samplesexposed to UV radiation in the environment, the samples were placed in aUV chamber and kept for 4 weeks. The ice adhesion before and after UVexposure remained unchanged, as shown in FIG. 10D.

Mechanical, chemical and environmental durability of the developedicephobic materials were also examined. The mechanical durability of theicephobic coatings was examined through Taber abrasion test (TaberReciprocating Abraser, Model 5900) according to ASTM D4060. In theseexperiments, material removal for different samples as various loadingconditions (i.e. 1, 5, and 10 N) was measured. Samples were placedfirmly on a horizontal plate in the Taber instrument and 1000 abrasioncycles applied in each experiment. Superhydrophobic surfaces and SLIPSfailed all the tests. AI-10 (67 % phase II concentration) failed the 10N abrasion test. However, other AI samples passed the tests in allloading conditions. The thickness removal in the abrasion tests areshown in FIG. 11A. After abrasion test under 5 N loading for 1000cycles, the icephobic performance of coatings exposed to mechanicalloadings was re-examined. The ice adhesion for these samples along withstate-of-the-art icephobic surfaces are shown in FIG. 11B. As shown, nomeasurable change in ice adhesion was observed and the AI samplesoffered minimal ice adhesion. In contrast to surface-modified materials(i.e. superhydrophobic surfaces or hydrated-surfaces), thestress-localization property of these materials is volumetric and doesnot change as they abrade. This feature ensures low ice adhesion onthese stress localized viscoelastic surfaces for long-term performance.As another metric for its mechanical durability, the icephobic coatingwas abraded through sand paper and iron file. No change in itsproperties was measured. The coating holds its low ice adhesion as theicephobic characteristics is a volumetric property and not a surfaceproperty.

Depending on the application, the icephobic coatings may be exposed tovarious chemical environments. The chemical stability of the AI coatingswas examined in a range of solutions with pH between 1-13. The acidicsolutions were prepared through various HCl and water concentrations.The basic solutions were Tris 0.15 mM NaCl (pH=8) and Sodium hydroxide(pH=13) solutions. The samples were soaked in these solutions for 48hrs. There was no change in the integrity of the coatings after beingexposed to these chemical environments. No change in the ice adhesion onthese coatings after chemical stability test was detected. To assessenvironmental durability of icephobic coatings, the samples were testedfor UV radiation effects. The icephobic sample was placed in a chamberfor 500 hours under UV radiation. No cracks or material degradation orchanges to the material’s durability were spotted. After UV exposure,the icephobic coating was re-examined under abrasive loading of 5 N. Theamount of material removed from the coating remained the same as beforeUV radiation. That is, the integrity of the coating is not affected byUV radiation. Finally, to demonstrate on-field repairability of thiscoating, the coating was damaged with a sharp blade to remove a part ofmaterial. The coating was then repaired by spraying of a new coating.The newly sprayed icephobic material was integrated within the coatingand no visible change in the coating was observed. The repaired surfacekept its integrity and icephobic properties.

In aerospace applications, icephobic coatings should have minimal effecton the aerodynamic characteristic of the airfoil (i.e. drag and lift).To examine these characteristics, a wing with a cross section close toNACA 6415 airfoil profile was chosen. The experimental setup includedtwo wing sections, which were removed from a small, commerciallyavailable wind turbine (ALEKO Vertical Wind Power Generator) in whichthey were used as the turbine blades to generate torque for a smallgenerator. Of the two wing sections, one was coated with an example ofthe icephobic coating and the other one was left uncoated. Beforeconducting any experiments, the lift and drag coefficients wereestimated for different angles of attack using XFOIL, a programdeveloped to analyze subsonic isolated airfoils. XFOIL analyzes the 2Dairfoil profile of a NACA 6415 under viscous flow conditions with aReynolds number of 90,000 and a Mach number of 0.09 to compute the liftand drag characteristics of the airfoil. The mounting system wasdesigned using Autodesk Inventor and was tailored specifically for usewith the NACA 6415 cross-section and the 6-Axis load cell. The mountingsystem consisted of an airfoil mount and two circular plates as part ofthe load cell mounts, one of which was fixed to the base of the windtunnel and the other was fixed to the bottom of the load cell. The twoload cell plates were designed in such a way that the top plate couldrotate on top of the bottom plate, with increments of 1°, covering thecomplete 360° range. This design feature was used to change the angle ofattack of the wing section attached to the load cell. The plates werealso designed to have 360 holes so that the plates could be pinned tohold the testing system at a certain angle of attack.

After the CAD drawing was made, the mounting system was 3D-printed usingPLA (Polylactic Acid) filament with a 100 infill to provide structuralrigidity. Each wing section was attached to a 6-Axis load cell in thewind tunnel, which in turn was attached to the base of the wind tunnel.The 6-Axis load cell measured the forces and torques acting on thesurface of the load cell and had a left-handed coordinate system. Thewings were placed in a recirculating wind tunnel with a rectangular testsection with a cross-section measuring 1.05 m × 1.65 m. The wings weretested at a constant wind speed of 17 m/s, so as to match the conditionsused in XFOIL, which corresponds to a chord Reynolds number ofapproximately 50,000. The forces and torques acting on the wing weremeasured simultaneously by the load cell for a given angle of attack.The force and torque measurements were used to determine the 2D drag andlift curves for the airfoil, with and without the icephobic coating.

The experimental setup is shown in FIG. 12A. The lift and dragcoefficients for the coated and uncoated wing sections were plottedagainst angle of attack in FIG. 12B and FIG. 12C. Furthermore, the ratioof lift/drag versus of angle of attack is plotted in FIG. 12D. Theexperimental data sets for both coefficients of lift and drag areaccompanied by error bars, which were calculated based off theresolution of the load cell. The XFOIL data sets do not have anycorresponding error bars, since this was a computational value. Theresults indicate that lift and drag for the coated wing and the uncoatedwing have a similar trend for different angle of attacks and thedifference in magnitudes on both is small. The magnitude of lift anddrag coefficients of the airfoils found experimentally differs from theXFOIL computational results because XFOIL is a 2D computational toolthat does not account for three dimensional effects, such as the 3Dcharacteristics of the finite wing. In a finite wing, thehigher-pressure air from beneath the wing tries to move towards thelower pressure above the wing. Moreover, the new experimental dataindicate that the coating does not affect the lift and dragcharacteristics of a wing, which is important in any passive alternativefor deicing aerospace systems.

To demonstrate the role of stress localization function on ice adhesion,an experimental procedure was designed to probe crack nucleation at thematerial-ice interface. A form of the icephobic material was developedand was applied to a glass substrate. The coating included PDMS matrixand black organogel particles to provide contrast for visualization ofcrack nucleation at the material-ice interface. Organogel particles wereincluded at 5% concentration according to volume in the PDMS matrix. Thedimension of the organogel particles was between about 100 nm and 20microns. A silanized glass prism (15 mm × 15 mm × 25 mm) was placed onthe icephobic material to resemble interaction of ice with the coating.The glass slide was placed on a moving stage, the movement of which iscontrolled by a motorized motion controller and computer. The motorizedstage was a syringe pump with forward velocity variation of 0.5 µm/s to5 mm/s. A firmly held beam load cell (Imada, model DS2-110) was used tomeasure the force. The force was applied at a distance of 1 mm above theinterface. The interface of the icephobic material-prism was viewed asshown in FIG. 13A. Through a coupled optical microscope and a high-speedcamera system, the crack nucleation at the interface was probed. FIG.13B shows micrograph of interfacial cracks observed during theseexperiments. As shown, all the interfacial cracks were formed at thecoordinate of phase II particles having low shear modulus. That is,phase II particles were responsible for cavitation and crack initiationat the interface. The fringes observed at the crack coordinatesindicated the ellipsoidal form of these cavities. The generated crackinduces a local stress field and the stored elastic energy depends onshear modulus of phase I and the dimension of these cracks. This storedelastic energy leads to a shear force at the front of crack, propagationof crack, and detachment of ice from the material.

To determine the value of the stress-localization function for examplesof the stress-localized icephobic materials, the values of W _(a) , thework of adhesion of the material, and G_(m), the shear modulus of thematerial, were determined. Work adhesion is determined as where y^(w)denotes surface tension of liquid (i.e. water) at -20° C. and θ is thecontact angle of sessile droplet on the surface. The contact angle ofwater was determined for the various samples and the work of adhesionwas consequently determined. The shear modulus of the example materialswas also measured using a Dynamic Mechanical Analyzer (DMA). Themeasured values are shown in Table 2 below.

Table 2 Phase I Phase II AI-10 AI-11 AI-12 AI-13 W _(a) (mN/m) 48 70 5753 51 49 G_(m) (MPa) 3.5 +/- 0.5 NA 0.6 +/- 0.5 0.9 +/- 0.5 1.4 +/- 0.51.8+/- 0.5

Using the figures in Table 2, the values of the stress localizationfunction were determined and plotted as shown in FIG. 13C. The stresslocalization function depends on the concentration of phase II in thematerial structure as predicted. This stress-localization functionreduces ice adhesion on the icephobic material up to an order ofmagnitude. The role of the stress localization function on reduction ofice adhesion is several times higher than the role of shear modulus. Forexample, comparing the sample AI-10 and pure silicon elastomer, thedifference of shear modulus is approximately six times which results in~ 2.5 times reduction in ice adhesion. However, for the same samples,stress localization reduces the ice adhesion by more than 12 times. Thestress localization function depends on geometrical parameters (a and l)along with volumetric fraction of phase II. For high values of a/l therole of normal force is dominant in the fracture and the role of stresslocalization (i.e. shear force) is small. However, for low values ofa/l, the fracture is governed by shear forces and the stresslocalization is the dominant factor. The developed physic ofstress-localization is applicable in detachment of any solid material(ice, dust and even bio-species) from elastomers.

1. A surface having anti-icing properties, comprising: a surface; and aviscoelastic icephobic coating deposited on the surface, wherein theviscoelastic icephobic coating comprises an elastomer matrix andorganogel particle beads dispersed throughout the elastomer matrix,wherein the organogel particle beads comprise a non-crosslinked liquidphase entrapped in a three-dimensionally crosslinked gel, wherein theelastomer matrix is cured to form the viscoelastic icephobic coating,and wherein the viscoelastic icephobic coating imparts anti-icingproperties to the surface.
 2. The surface of claim 1, wherein theelastomer matrix comprises polyurethane, poly isoprene, silicone rubber,or combinations thereof.
 3. The surface of claim 1, wherein theorganogel particle beads comprise organogels, polyacrylamide,polydimethylsiloxane, or combinations thereof.
 4. The surface of claim1, wherein the organogel particle beads comprise one or more siloxanes,one or more silicas, ethyl benzene, or combinations thereof.
 5. Thesurface of claim 1, wherein the organogel particle beads comprisedimethyl siloxane, dimethylivinyl terminated silica, dimethylvinylatedsilica, trimethylated silica, tetra (trimethoxysiloxy) silane, ethylbenzene, dimethyl methylhydrogen siloxane, tetramethyl tetravinylcyclotetra siloxane, or combinations thereof.
 6. The surface of claim 1,wherein the organogel particle beads are about 10 nm to about 100microns in diameter.